The present invention relates to security documents including a security image composed of individual security image elements and complementary security image elements. When an attempt is made to duplicate or reproduce a document on which the security image is present, at least some of the security image elements change their appearance on the intended duplicate document, or are altered with respect to some other physical characteristic on the intended duplicate or reproduced document. The change in appearance, or the altered physical characteristic, of the elements is utilized to provide an indication that an attempt has been made to duplicate the document.
For example, the security image elements and the complementary security image elements may be designed such that, upon the attempted duplication, e.g., by photocopying, either the security image elements or the complementary security image elements are altered while the others remain substantially the same. The elements are arranged such that the attempted duplication results in the formation of a readily apparent warning image on the face of the document. Specifically, when an attempt is made to copy the document, the word "VOID" may appear on the duplicate document. Further, the security image elements and the complementary security image elements are arranged such that the presence of the security image and the security image elements is not readily apparent on the original. Examples of security documents of this type are illustrated in U.S. Pat. Nos. 4,579,370, 5,149,140, 5,197,765, 5,340,159, the disclosures of which are incorporated herein by reference.
Accordingly, the above-described prior art provides a means by which a person charged with the responsibility of reviewing security documents can identify documents that are unauthorized copies of the original security document. However, the above-described conventional security documents are not specifically designed so as to be reliably verifiable by an electronic document reader, e.g., an optical imaging device. Accordingly, there is a need for a security document, a method of preparing a security document, and a security document processing system wherein the security document is specially designed to provide a reliable indication of authenticity to an electronic authentication device without compromising the document's ability to provide a human-recognizable indication of authenticity.